To determine the strength of a rock formation and its capabilities in containing pressure, several geomechanical parameters must be known. These parameters include but are not limited to: Young's modulus, which is a measure of the stiffness of an elastic material, cohesion, the component of rock shear strength that is independent of inter-particle friction, Poisson's ratio, which represents the negative ratio of transverse to axial strain, and minimum in-situ stress, which is the amount of pressure it takes to initiate the opening of an existing fracture. The conventional method of obtaining these values is to perform a mini-frac and to test core samples at an offsite lab. However, these procedures are very costly and take a very long time. Furthermore, core samples are inherently disturbed during the coring process and thus may not accurately represent the in-situ rock conditions.
There have thus been various instruments developed for testing geomechanical properties of in-situ rock formations. For example, pressuremeters are sometimes used to determine minimum in-situ stress and borehole shear testers are sometimes used to test rock shear strength.
However, conventional pressuremeter tools are generally unable to provide reliable minimum in-situ stress values due to the difficulty in pinpointing the initial onset of a crack in a formation. Also, while conventional pressuremeters can determine many geomechanical strength properties, they cannot provide sufficient data to determine cohesion. Conventional pressuremeters are similarly unable to determine permeability (the measure of a rock's ability to transmit fluids) due to the need to inject fluid or gas into a rock, which conventional pressuremeters cannot do. Presently-available self-boring pressuremeters allow for fluid movement through the tool itself, but cannot exert the force required for all sought-after testing functionality, both on the injection and cavity expansion aspects of their testing.
Borehole shear testers are another instrument that can be used in testing geomechanical properties. They are typically one dimensional tools that can provide only partial answers to the questions surrounding geomechanical behavior. For the most part, they are limited in their normal force exertion capability, functioning generally in soft soil only. Conventional borehole shear testers are also rigid in their design and occupy the entire wellbore when deployed.
Presently-available in-situ rock testing equipment thus does not provide comprehensive information regarding formation characteristics. As a further example, thermal hardening of a material is a response which can help increase that material's resistance to failure and conventional means of testing for the effects of thermal hardening involve high temperature tri-axial testing, which is currently unavailable using presently-available in-situ tools and methods.